That said, this looks like something I'd definitely use, or at least experiment with in C# if I was still doing work in the language - though for the actor bits I would probably lean on Orleans.
Author here. That is simply not correct. It has a full supervision hierarchy as well as a compositional strategy system for creating policies that handle failures. The strategies can also be scripted as part of the config, to detach the failure handling entirely from the actor itself.
I couldn't just jump ship and use F#, Haskell, or Erlang; so I decided to bring what I'd learned from those languages into C#, and create a library that will help me write better C# code.
My mission is to create a library that reduces the cognitive burden of writing C#. It isn't a functional manifesto for C#, it just looks like it because I've found that's the best way to achieve my goals.
The Process system (Erlang style actors) is there to deal with the 'edges' of functional code. It's not possible to be pure like in Haskell, and so you will have stateful code - with all the problems that come with it, i.e. shared memory, locks, mutability, etc. Actors allow a certain amount of control that you don't get from C# classes: Single-threaded and no-shared state. They can be distributed anywhere and the mechanism for interaction is the same. You can build in routing or proxying to do load-balancing, etc.
The thought of trying to fix up an old code base did not even cross my mind. Probably because that is what my nightmares are made of.